Rampant Sargassum

Volunteers remove the weed from the shore. Video: B-onair


Sargussum washing up on the shorelines of the Caribbean is causing huge damage. What can we do about it? WUR researchers are looking for solutions in the lab and on location. Sargassum may not be easy to deal with, but it does have potential for beneficial applications, such as sustainable fuel.

Something strange is going on in the Caribbean. Since 2011, vast quantities of brown seaweed from the Atlantic Ocean have been washed up on the coast of places including St. Maarten, Bonaire, Mexico and Florida very regularly. These seaweeds — generally known by their Latin name, the genus Sargassum — cause huge damage to plants, animals and humans. “Sargassum are seaweed species that float on the ocean surface,” explains Ana López Contreras, a researcher at Wageningen University & Research. “It looks like a kind of carpet, a mat on the water. That mat can expand to form a floating colony, kilometres in length.”

Massive Sargassum fields on sea. Photo: Bret Reyes / Shutterstock

This is not a problem, as long as the mats remain far out on the ocean or if the seaweed only washes ashore in small quantities. “Quite the opposite, in fact,” says marine ecologist Matthijs van der Geest of Wageningen Marine Research, who studies the seaweed along with López Contreras’ team. “In the open ocean, the floating Sargassum mats provide essential habitats for a wide range of fish, sea turtles, seabirds and other animals. They find food and shelter among the seaweed. The floating mats thus have a beneficial effect on biodiversity and productivity in open-ocean waters, where normally little food and living space can be found. When washed up on the beach, Sargassum adds nutrients to the soil and helps prevent erosion.”

Ecological disaster

That’s the upside. The downside is that the amount of Sargassum seaweed in the Atlantic has drastically increased since 2011. And even more worryingly, that mass of seaweed is floating towards the shore. “Depending on the winds and ocean currents, it washes up in the Caribbean or the United States,” says Van der Geest. “Millions of tons of organic material end up in shallow coastal waters and on beaches. It’s causing an ecological disaster.” It’s still unclear why so much Sargassum has suddenly appeared in the Atlantic in the past decade, although there are theories. Deforestation and climate change mean that the Atlantic waters contain far more nutrients nowadays, which lets Sargassum grow faster. Rising ocean temperatures may also be at play.

Sargassum (seaweed) is a breeding ground for young fish and turtles, but once ashore it mostly breeds problems. This holds true for the Netherlands Antilles, where researchers are investigating sustainable solutions, such as turning it into biofuel for electricity and fertiliser.

One thing is certain: the seaweed causes a lot of problems when it ends up on shorelines. That’s why a team of WUR scientists is investigating what can be done to reduce the negative impact of large quantities being washed ashore. What problems does the seaweed cause exactly when it washes up? On beaches, Sargassum piles up to form mountains of brown biomass. When that starts to rot, it releases hydrogen sulphide, a toxic gas that stinks of eggs that have gone off. Hydrogen sulphide can cause sickness and even death in plants, animals and humans.

In the open ocean, the floating Sargassum mats provide essential habitats for a wide range of fish, sea turtles, seabirds and other animals

But the brown seaweed can cause huge damage even before it starts piling up on the beach. Seagrasses and coral are often found in the clear coastal waters of the tropics. “Sargassum forms a thick, dark blanket across the water surface,” explains Van der Geest. “So the seagrasses and coral living on the seabed no longer get any light, which inhibits their growth.”

Coral under threat

The issues only get worse. As the Sargassum accumulates in the shallow coastal waters, the seaweed itself gets less and less sunlight. Van der Geest: “So it starts to rot, sinks to the bottom and ends up on top of the coral and seagrass, which then get damaged and die off — whereas precisely these structures are essential for sea life.”

Dried Sargassum. Photo: Wageningen Food and Biobased Research

Furthermore, the rotting Sargassum uses up all the available oxygen, making the soil and water column devoid of oxygen and killing off soil life. Fields of seagrass and coral reefs are important places of shelter for small marine creatures. Fish can lay their legs there and the seagrass is a source of food for turtles. “If all that goes lost, you’ll also lose an awful lot of biodiversity that you can’t bring back easily. Moreover, coasts without seagrasses and coral reefs erode faster and are more vulnerable to hurricanes.”

Fewer tourists

It’s clear that coastal life is suffering from the rampant growth of the seaweed. The local people are affected too — and not just by the horrible smell. The economy of the Caribbean islands depends heavily on tourism and fishing, both of which are inhibited by the Sargassum. “Fishing boats and their nets get caught up in the seaweed,” explains Van der Geest. “And you have to admit that if you go on holiday to a tropical island, the last thing you want is a beach covered in rotting seaweed — especially if you can no longer go snorkelling, diving or surfing and if there is hardly any sea life left.” What can we do to keep the fast-spreading Sargassum in check? It certainly isn’t easy: “The volumes are often too huge to rake up by hand,” says project manager López Contreras. “Of course you could use bulldozers to haul up the seaweed that has washed up, but that will also damage the beach. The best solution is to prevent the seaweed from washing ashore in the first place, for example by harvesting it at sea.”

Sargassum inhibits tourists of Bonaire from swimming or snorkeling. Photo: Stefan Kogelman / Shutterstock

Preferably about one kilometre off the coast. Not in the open ocean, stresses ecologist Van der Geest, because the Sargassum serves an important purpose there as a nursery for new life. “And not on the beach either, because at that point the seaweed has already devastated the coastal system.” Another option is to set up a barrier of floating hooks along vulnerable coastal areas so that the Sargassum piles up there instead. “But you’ll still have to harvest the seaweed in time so that it doesn’t break the barrier after all.”

Satellite images

Scientists are trying to stay one step ahead of the Sargassum with the help of technology. Sargassum blooms are visible in satellite images, floating with the ocean currents. “So you can track them remotely,” says López Contreras. “We want to use this information to predict when and where exactly the seaweed will wash ashore so that we can intercept it in good time.” Such an early warning system will help to place barriers in the right place and harvest the seaweed in a targeted way. But both researchers say that more work needs to be done on the impact that such interventions have on the environment. For example, there is the risk of bycatch of creatures that live among the seaweed when harvesting at sea, and the risk of the hooks impeding the migration of young turtles if they are not installed carefully.

If we fail to act now, we will eventually have an even bigger ecological and economic problem

López Contreras: “When we started this research, I thought it would be simple — just harvest the seaweed and there you have it. But there is an equilibrium in nature that we mustn’t disrupt. It is vital that we study what consequences our solutions can have before we take action. Obviously we want to make sure we don’t just create an even bigger problem.”

Sustainable fuel

Regardless of whether you harvest the Sargassum or just collect it first, what do you do with all that biomass? “The seaweed naturally contains relatively high concentrations of heavy metals,” says Van der Geest. “You can’t simply feed it to goats. If you let it rot on land, the toxic substances may end up in the groundwater.” Fortunately, there is one promising solution: convert it into energy. On the WUR campus, López Contreras’ team is investigating how you could turn Sargassum into a sustainable fuel. “That has huge potential. The Caribbean islands still import a lot of their fuel, so if you could generate electricity from local biomass that is present in excess, it would make the communities there more self-sufficient.”

Volunteers remove the weed from the sea. Photo: S. Engel

The yield: bags filled to the brim. Photo: Shutterstock

That’s theoretically possible: biogas or biofuel has been produced in the lab from other kinds of seaweed that are similar to Sargassum. The Sargassum could be mixed with other biomass such as household organic waste to generate biogas. “But we need to do more research before such technology can be used on a large scale. It may be possible in five to ten years — provided there is sufficient funding.”

Universal problem

Despite all the technology, it remains difficult to predict how much Sargassum will wash ashore, and when and where. Taking dedicated measures to prevent coastal areas from being ravaged requires a proper understanding of why the seaweed has grown so much in the past ten years. “This isn’t something happening to a few small islands,” emphasises López Contreras. “It’s a problem on a global scale. We’re talking about thousands of kilometres of Caribbean coastline that is under threat — and Sargassum is also starting to wash up in West Africa. If we don’t do anything, it may eventually end up in Europe.” She finishes by stressing how important it is for governments to take action too, including in the Netherlands. “Because if we fail to act now, we’ll soon have an even bigger ecological and economic problem.”

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